How can I transfer everything from my original phone to the extra phone that I’m using without losing information on either phone?
That's not possible, if you are hoping to preserve what is already on the extra phone and add what was on the original phone. This would be "merging" the data of both phones on the extra phone.....which is not possible.
Assuming you have a current backup of your original phone on iCloud or a computer, you can erase the extra phone and then set up the extra phone using the backup of your original phone.
The extra phone must be running the same or higher iOS version as your original phone.
Then, your extra phone will look just like your original phone.
You cannot "merge" the data of both phones. Whatever is on the extra phone will be erased and replaced by the backup of your original phone.